5W30 and 5W40 oils behave similarly when the engine is cold, but differ in viscosity at operating temperature: 5W40 is thicker and forms a thicker oil film, while 5W30 is thinner and provides less resistance. Both designations are based on the SAE J300 classification and refer only to viscosity, not oil quality. Oil selection is determined by the vehicle's owner's manual and manufacturer standards.
To sum it up
- Designation «5W» is common - both oils have the same requirements for operation in winter conditions, so they pump equally well when starting the engine in cold weather.
- At 100°C The viscosity of 5W30 oil is 9.3–12.5 mm²/s , A 5W40 – 12.5–16.3 mm²/s (SAE J300).
- 5W30 oil helps improve fuel economy and often meets low SAPS standards for engines with diesel particulate filters; 5W40 oil is better able to withstand high thermal loads.
- More important than viscosity are ACEA/API standards and manufacturer approvals (e.g. VW, MB, BMW) listed in the manual.
What do the symbols 5W30 and 5W40 mean?
The viscosity of motor oils is determined by the American standard SAE J300 . All-season oil consists of two parts. The first, followed by a letter W (from winter - winter ), describes the properties at low temperatures. The second number, after the hyphen, describes the viscosity at the engine's operating temperature.
For the 5W grade, the standard requires the oil to be able to "rotate" in a test simulating starting at -30°C and be pumpable at -35°C. Since 5W30 and 5W40 oils have the same winter service life, they exhibit comparable freezing characteristics . The difference lies in the second indicator.
The difference between 5W30 and 5W40 oils is viscosity at operating temperature.
The SAE J300 standard sets a range for each class of summer tires. kinematic viscosity at 100 °C and minimal HTHS viscosity , measured at 150 °C under high shear conditions, i.e. under conditions similar to those in shaft bearings or camshafts.
- SAE 30 (5W30): from 9.3 to less than 12.5 mm²/s at 100°C, HTHS not less than 2.9 mPa s.
- SAE 40 (5W40): from 12.5 to less than 16.3 mm²/s at 100°C, HTHS not less than 3.5 mPa s.
In practice, this means that 5W40 oil becomes thicker after warming up. A thicker oil film provides better protection during long highway trips, towing a trailer, or sporty driving. At the same time, the thinner 5W30 oil reaches friction points faster and creates less internal resistance, resulting in slightly lower fuel consumption and emissions.
Is 5W30 or 5W40 oil suitable for gasoline?
Fuel type alone does not determine viscosity selection. For many modern gasoline engines, especially those with direct injection and turbocharging, 5W30 oils (or even thinner ones) are recommended, as these engines are designed for fuel economy and emission compliance. For many older gasoline engines and some high-performance units, 5W40 is recommended—often with an ACEA A3/B4 rating, indicating a high HTHS viscosity for engines operating under severe conditions.
The key question, therefore, is not "petrol or diesel" but which oil is allowed by a specific engine manufacturer . If the manual specifies different viscosity grades, 5W30 is generally the best choice for fuel economy, while 5W40 is better for heavy-duty operation or high ambient temperatures. It's important that the oil meets the required quality standards and, if specified, the manufacturer's approval.
Why is 5W30 oil more expensive than 5W40?
Viscosity alone doesn't determine price. However, 5W-30 oil often costs more on store shelves because oils with high technical characteristics more commonly sold under this designation: ACEA C2 or C3 standards (the so-called low SAPS oil , with reduced ash, sulfur, and phosphorus content, safe for diesel particulate filters (DPF/GPF) and automaker approvals for long-life oils. These oils require synthetic base oils and more expensive additive packages, and obtaining approval requires extensive testing.
On the other hand, 5W40 is a very common class of oils, produced in large quantities, even in versions with "classic" additive levels (e.g., ACEA A3/B4). Therefore, when comparing prices, it's best to compare oils with the same standard and base stock—then the differences usually disappear. The cheapest oil with the correct viscosity, but without the required standard, doesn't mean you're saving money.
Comparison table: 5W30 vs. 5W40
| Characteristic | 5W30 oil | 5W40 oil |
|---|---|---|
| Winter course | 5 W – starts at -30°C, pumping capability at -35°C | 5W – identical requirements |
| Viscosity at 100 °C | 9.3–12.5 mm²/s | 12.5–16.3 mm²/s |
| Minimum heat exposure temperature at 150°C. | 2.9 mPa s | 3.5 mPa s |
| A film of hot oil | Thinner, less resistance | Thicker layer, more reliable protection |
| Fuel consumption | A little lower | A little higher |
| Typical standards | ACEA C2/C3 long-lasting oils are often used. | ACEA A3/B4 and C3 are common. |
| Typical use | Newer engines, diesel particulate filter (DPF/GPF), economical driving mode. | Engines under heavy load, some older models. |
Which oil to choose? Practical advice.
- Start with the user manual – there you will find the permissible viscosity values, ACEA/API standards and any manufacturer standards (e.g. VW 504 00/507 00, MB 229.51).
- For vehicles with a diesel particulate filter (DPF or GPF) ) Choose low SAPS (ACEA C) oil if recommended by the manufacturer; regular oil may accelerate filter clogging.
- Heavy highway driving, trailer towing, hot weather – if the operating manual allows the use of both viscosity types, then 5W40 oil will provide a greater margin of protection.
- Mainly for the city and short trips – if the manufacturer permits, 5W30 oil helps reduce fuel consumption.
- The engine started consuming oil – Do not change its viscosity yourself; consult a mechanic, as the cause may be wear of components.
Frequently Asked Questions (FAQ)
Can I use 5W40 oil instead of 5W30?
Only if the engine manufacturer allows this viscosity and the oil meets the required standards. Many manuals list both options, but some modern engines (especially those designed for long service life) recommend using only oil with a lower viscosity. If in doubt, check the manual or contact a service center.
Can you mix 5W30 and 5W40 oils?
In an emergency, adding a small amount of oil of a different viscosity but meeting the specified specifications is usually harmless—the mixture will have an intermediate viscosity. However, this is not a permanent solution; at the next oil change, top up with oil that meets the manufacturer's recommendations.
Which oil is best for winter?
Both models have the same 5W winter program, so they behave similarly when starting in cold weather. The difference only becomes apparent after the engine warms up.
Is 5W40 oil suitable for a high mileage engine?
This is often recommended, as a thicker oil film can provide better performance with larger clearances. However, this isn't a universal rule—if the manufacturer only recommends 5W30 oil, consult a mechanic before changing the viscosity.
Does higher viscosity mean a better oil?
No. Viscosity only indicates the oil's consistency at a given temperature. Quality and suitability for engine use are determined by ACEA/API standards and automaker approvals.
See also
- What is the difference between 5w40 and 10w40 oils?
- What is the difference between 75w80 and 75w90 oils?
- What is the difference between synthetic and semi-synthetic oil?
- What is the difference between 95 and 98 octane gasoline?